1. Field of the Invention
The present invention relates to thin film transistors.
2. Description of the Related Art
As one kind of electric field effect transistors, there is a known thin film transistor in which a semiconductor layer provided over a substrate having an insulating surface is used for a channel formation region. Techniques by which amorphous silicon, microcrystalline silicon, and polycrystalline silicon are used for semiconductor layers of thin film transistors have been disclosed (see Patent Documents 1 to 5). Thin film transistors have been used, for example, for liquid crystal television devices and put into practical use as switching transistors for pixels of display screens thereof.    [Patent Document 1] Japanese Published Patent Application No. 2001-053283    [Patent Document 2] Japanese Published Patent Application No. H5-129608    [Patent Document 3] Japanese Published Patent Application No. 2005-049832    [Patent Document 4] Japanese Published Patent Application No. H7-131030    [Patent Document 5] Japanese Published Patent Application No. 2005-191546